feat(security): A-1 Ed25519 key rotation + A-2 replay attack fixes

Complete RedFlag codebase with two major security audit implementations.

== A-1: Ed25519 Key Rotation Support ==

Server:
- SignCommand sets SignedAt timestamp and KeyID on every signature
- signing_keys database table (migration 020) for multi-key rotation
- InitializePrimaryKey registers active key at startup
- /api/v1/public-keys endpoint for rotation-aware agents
- SigningKeyQueries for key lifecycle management

Agent:
- Key-ID-aware verification via CheckKeyRotation
- FetchAndCacheAllActiveKeys for rotation pre-caching
- Cache metadata with TTL and staleness fallback
- SecurityLogger events for key rotation and command signing

== A-2: Replay Attack Fixes (F-1 through F-7) ==

F-5 CRITICAL - RetryCommand now signs via signAndCreateCommand
F-1 HIGH     - v3 format: "{agent_id}:{cmd_id}:{type}:{hash}:{ts}"
F-7 HIGH     - Migration 026: expires_at column with partial index
F-6 HIGH     - GetPendingCommands/GetStuckCommands filter by expires_at
F-2 HIGH     - Agent-side executedIDs dedup map with cleanup
F-4 HIGH     - commandMaxAge reduced from 24h to 4h
F-3 CRITICAL - Old-format commands rejected after 48h via CreatedAt

Verification fixes: migration idempotency (ETHOS #4), log format
compliance (ETHOS #1), stale comments updated.

All 24 tests passing. Docker --no-cache build verified.
See docs/ for full audit reports and deviation log (DEV-001 to DEV-019).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

aggregator-server/internal/api/middleware/auth.go

@@ -0,0 +1,71 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/google/uuid"
+)
+
+// AgentClaims represents JWT claims for agent authentication
+type AgentClaims struct {
+	AgentID uuid.UUID `json:"agent_id"`
+	jwt.RegisteredClaims
+}
+
+// JWTSecret is set by the server at initialization
+var JWTSecret string
+
+// GenerateAgentToken creates a new JWT token for an agent
+func GenerateAgentToken(agentID uuid.UUID) (string, error) {
+	claims := AgentClaims{
+		AgentID: agentID,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(24 * time.Hour)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString([]byte(JWTSecret))
+}
+
+// AuthMiddleware validates JWT tokens from agents
+func AuthMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authHeader := c.GetHeader("Authorization")
+		if authHeader == "" {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "missing authorization header"})
+			c.Abort()
+			return
+		}
+
+		tokenString := strings.TrimPrefix(authHeader, "Bearer ")
+		if tokenString == authHeader {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid authorization format"})
+			c.Abort()
+			return
+		}
+
+		token, err := jwt.ParseWithClaims(tokenString, &AgentClaims{}, func(token *jwt.Token) (interface{}, error) {
+			return []byte(JWTSecret), nil
+		})
+
+		if err != nil || !token.Valid {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid token"})
+			c.Abort()
+			return
+		}
+
+		if claims, ok := token.Claims.(*AgentClaims); ok {
+			c.Set("agent_id", claims.AgentID)
+			c.Next()
+		} else {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid token claims"})
+			c.Abort()
+		}
+	}
+}